Background: In their new one-pool correction equation (the JBM equation) for slope-intercept measurement of glomerular filtration rate (GFR), Jodal and Brochner-Mortensen (JBM) assumed that immediate tracer distribution volume (Vo) is the plasma volume. Derivation of the correction factor, f, required the measurement of plasma volume and the area, a, 'missing' in the slope-intercept technique. However, this study shows that any tracer distribution volume can be substituted for Vo to measure f.
Methods: Extracellular fluid volume (ECFV) and GFR were measured in 69 childhood patients and 101 renal transplant donors using chromium-51 (Cr-51)-EDTA. Data from 20 normal volunteers, in whom ECFV was measured with iohexol and GFR with Cr-51-EDTA, were used to measure a. Lean body mass and ECFV (ECFVBird) were estimated from equations based on height and weight described by Boer and Bird et al., respectively. Body surface area (BSA) was estimated from Haycock's formula.
Results: In children and donors, ln ECFV correlated closely with ln BSA [ECFV (millilitre) = 6080 × BSA; R = 0.943]. In normal volunteers, a was 18.6 (SD 5.5) min, resulting in f = 0.00308 × BSA min/ml, similar to the value of f in the JBM equation (0.0032 × BSA min/ml). BSA correlated closely with lean body mass (R = 0.80 in women and 0.98 in men). ECFVBird was almost identical to 6080 × BSA. GFR scaled to BSA correlated with BSA, but GFR scaled to BSA did not.
Conclusion: Any distribution volume proportional to Vo can be used to calculate f in the JBM equation. The unsuitability of BSA for scaling GFR as a result of their nonlinear relationship is shown.